Field of the Invention
The invention relates to the construction material industry and can be used for producing gypsum binders and products made on their basis.
Description of the Related Art
The principle of obtaining gypsum binders is based on a capacity of calcium sulfate dihydrate, when heated, to undergo dehydration accompanied by changes in the crystal lattice structure (re-crystallization).
Natural gypsum stone, recycled gypsum and synthetic gypsum, which is a co-product of the chemical, power, food, wood-chemical and other industries (phosphogypsum, borogypsum, chlorogypsum, cytrogypsum, etc.), are used as raw materials for production of gypsum binders.
The industry of low-baked gypsum binders, using a treatment temperature up to 900° C., produces several types of products: α- and β-modifications of calcium sulfate hemihydrate, dehydrated hemihydrate, soluble and insoluble anhydrite. Dehydrated hemihydrate and soluble anhydrite are unstable in the air and develop into hemihydrate. The majority of the production volume falls into α- and β-hemihydrates (α- and β-gypsum) or their mixtures. There is a market segment of insoluble anhydrites, though it is quite small.
Obtained by baking, α- and β-modifications of gypsum are characterized by the same crystal lattice type. Their difference is that the first has a better structure with well-defined large-size crystals, while the second consists of smallest aggregates of defective crystals with a developed inner surface. As a result, α-gypsum products show much better characteristics of strength and moisture resistance than β-gypsum products. However, the primary production volume of gypsum binders falls into β-hemihydrate, owing to its simpler and cheaper production technology.
The process for producing α-modification of gypsum can be subdivided into three key groups:                Heat treatment of lumpy gypsum raw material with saturated steam under pressure using various methods of drying the dehydrated product and subsequent milling;        Heat treatment of powdered gypsum raw material in aqueous suspension under pressure, with mechanical dehydration, drying, and milling;        Heat treatment of powdered gypsum raw materials at atmospheric pressure in salt solutions, followed by washing, dehydration, drying, and milling.        
The main disadvantage of the first two processes is that the processes require high pressurization and it is, normally, implemented only periodically. This significantly increases the cost of both the plant for their implementation and the resulting product.
The last two processes relate to processing in a liquid medium and use additional steps of washing and drying. This consumes a large amount of water and additional heat. This is a significant disadvantage of these methods. Therefore of great practical importance are methods of processing in a gaseous medium, eliminating the need for washing and further drying.
At temperatures 105-135° C., these processes provide, mostly, α-hemihydrate; at 200-210° C.—α-dehydrated hemihydrate; at 220-250° C.—α-soluble anhydrite. Higher temperatures produce insoluble anhydrite.
Construction gypsum (β-gypsum) is obtained using a simple technology by heat treatment of gypsum raw material at normal pressure within the temperature range 100° C.-160° C.
The processes can be subdivided into three key groups:                Baking lumpy raw material in drying drums or rotary kilns using flue gases at a relatively low rate of raw material dehydration and, as a consequence, with baking duration of several hours and flue gas components getting into the product;        Baking of pre-milled raw material in kettles with non-contact heating through the wall of a heating unit and baking duration not exceeding 2 hours;        Baking of milled raw material, contacting with heat medium, in a suspension (in mills, “fluidized bed” machines, etc.) at a high rate of the dehydration processes—up to 1 hour.        
The advantage of the processes for producing β-modification of gypsum is the technological simplicity that does not require high pressurization and low cost. Nevertheless, β-modification of gypsum, obtained using these methods, is characterized by poor strength properties, no moisture resistance and short mixing time.
In order to improve gypsum binder properties, depending on required characteristics, multi-phase (i.e., from diverse modifications of gypsum) and/or composite (with adding external non-gypsum components) gypsum-based mixtures are produced. Their production involves varying the composition, granulometry, and the component ratio for regulating functional properties: durability, setting time, water resistance, etc. There are great many various additives affecting properties of gypsum binders. All this greatly increases the cost of the finished product.
Known is a method of making the gypsum binder described in RU patent No. 2023699 published on Nov. 30, 1994, which suggests a method of producing a binder with predominating calcium sulfate α-hemihydrate. The method consists of gypsum dehydration carried out in lumps at atmospheric pressure, in a gaseous medium its important advantage is in use of an electromagnetic super-high-frequency field (SHF field). Due to volumetric heating affected by SHF radiation, dehydration with development of predominantly α-modification of gypsum occurs in the proper volume of a lump, which becomes a sort of “mini autoclave” for itself. β-modification of gypsum develops in the near-surface layer.
The disadvantage of this technical solution is applying, during its implementation, a very uneconomic method of heating with SHF radiation generated due to using electric power and, consequently, its high cost.